1. Field of the Invention
The invention relates to a method for installing cables in ducts using a fluid.
2. Description of the Prior Art
Such method, in which the fluid is a pressurized gas, is disclosed, e.g., in EP-A-0,108,590. Said method has the advantage of the propelling forces being evenly distributed over the length of the cable, so that installation is also possible in curved sections. In the event of conventional pulling of a cable, all force is concentrated in the initial part of the cable, as a result of which installation in curved sections is a problem.
A method of the above type, in which the fluid is water, is disclosed, e.g., in GB-B-2,122,367. Using fluid in general has the advantage that the friction between the outside cable wall and the inner duct wall is less than in the event of using air, since the cable floats, at least in part, in the fluid. A further advantage is that the capacity of the pump used for introducing the fluid into the duct, in the event of a liquid may be less than in the event of a gas, since a liquid has a higher viscosity than a gas. This is particularly advantageous for larger duct diameters. It may also be an advantage that the pressure drop in the event of using a liquid is linear, and there is therefore exercised a constant entraining force along the duct section. A still further advantage is that, in the event of using a liquid, electrostatic effects, as a result of which the cable is attracted to the duct wall, in most cases are capable of being cancelled out.
Using liquids, such as water, however, also has its drawbacks. In most cases, after installing the cable the water must be removed from the duct. In the event of optical-fibre cables without a metal water screen, there may occur fibre breakage in the presence of water as a result of stress corrosion. In the event of copper cables without water screen, the electrical properties will deteriorate. Finally, freezing of the water may cause damage. Another problem in the event of using water is that, in elevated parts of the duct section, particularly in duct sections featuring relatively steep parts, in a water-filled duct there may still remain enclosed an air bubble, as a result of which there arises additional friction in elevated parts. Having such air bubbles escape is possible, though expensive, which is also true for applying a vacuum to the duct in advance.
An object of the invention is to provide for a method in which the drawbacks of installing cables using a liquid are lacking or are suppressed to a great extent, while the advantages are being maintained.
For this purpose, the invention provides for a method of the above type, characterized in that a fluid is used which, under the operational pressure and operational temperature used during installation, is in a liquid state and which, at the ambient pressure and ambient temperature prevailing at the installation location, is in gaseous state.
The invention is based on the insight that, if use is made of a fluid which at the operational circumstances prevailing at the installation of cables in ducts by, e.g., varying the pressure and/or temperature, is brought to a liquid state and that at the ambient pressure and ambient temperature prevailing at the location of installation is in a gaseous state, the advantages of installing using a gaseous medium and installing with a liquid medium, may be advantageously combined. After all, a liquid is very suitable for installing, but hard to remove, while a gas has drawbacks in the event of installation but is simple to remove. Generally, for this purpose use may be made of a fluid which is in a liquid state at a preferably significantly lower pressure than the maximum permissible operational pressure, or due to a temperature reduction, but which upon reducing the pressure or increasing the temperature, is in a gaseous state once again.
The method according to the invention is particularly suitable to use in situations in which the installation duct bridges a difference in altitude, as in mountainous regions and in high-rise construction.
With the method according to the invention, it is possible to use a fluid, which is subject to such a pressure difference that it exercises an entraining force on the cable to be installed. It is also possible, however, to utilize the fluid exclusively to have the cable float and to further install the cable using a combination of pushing and pulling. In this case, the fluid either flows with a velocity lower than the propulsion speed of the cable, or the fluid may even flow in a direction opposite to the propulsion direction of the cable.
Examples of fluids for which the liquid state is attained substantially using a pressure increase are, e.g., HFKs (hydrofluorocarbons), ammonia, propane, butane, fluids which nowadays are used as cooling liquids and are, to a certain extent, environmentally safe. LPG (liquefied petroleum gas), tooxe2x80x94a mixture of propane and butanexe2x80x94may be used as a fluid. Another example are the CFCs [=chlorofluorocarbons) which, however, are less desirable in view of the damage incurred by the environment.
As an example of a fluid with which the liquid state is attained substantially by using temperature reduction, CO2 may be referred to. The advantage of CO2 is that it is neither flammable nor aggressive. In the event of using CO2, the pressure should preferably remain in excess of 5 bar (triple point) in order to prevent the formation of solid matter, which would impede the installation. At a normal operational pressure, the temperature should be significantly lower than the ambient temperature, e.g., xe2x88x9260xc2x0 C., in order to maintain the CO2 in the liquid state. At such a low temperature, the friction between cable and duct wall will decline. The density of CO2 is 1.1 g/cm3, somewhat higher than that of water, and makes it possible to exercise a great floating effect on a cable.
Table 1 offers an overview of examples of applicable fluids and their physical properties.
A second option, which is based on the concept on which the invention is founded, is, particularly in the event that the applicable fluid is recycled, to first have the duct in which the cable is to be installed blown through with the gaseous state of the fluid which later, at the installation proper of the cable, will be used in the liquid state. During the subsequent further filling of the duct with a fluid which is now, as a result of higher pressure and/or lower temperature, in the liquid state, the gas in the duct will also condensate to form a liquid. Due to said condensation of the gas in the duct, the problem referred to above of the inclusion of air bubbles is also obviated.
In addition, the invention comprises a device for carrying out the method according to the invention, provided with means to couple a fluid flow to the input end of the duct in order to fill the duct with the fluid, and with means to introduce the cable into the input end of the duct, characterized in that there are provided means to pressurize the fluid and/or reduce its temperature in order to bring the fluid to the liquid state, the means comprising a stock vessel and/or a compressor and/or cooling means, which compressor and/or cooling means are coupled to the input end of the duct.